This invention relates to the manufacture of structural foam thermoplastic boards which may be substituted for wood or composite molding which are presently used in the manufacture of picture frames and the like.
The vast majority of frames for pictures, mirrors and the like are presently manufactured by shaping an elongated wood board to a particular profile by milling, shaving, and other similar operations. As used herein, a board is defined as an elongated body of relatively little thickness, but of substantially greater width and very much greater length.
The elongated boards are decorated by several different methods. One method is to profile the entire board with a longitudinal design such as a groove or the like by a milling-like operation. Thus, the profile or cross-sectional shape along the entire length of the board is identical. A board profiled as described above can be used as is to form a frame or be further decorated.
Embossing by pressing or burning a design into the surface of a profiled board is one way to further decorate the board. Another technique by which a profiled board can be decorated is to apply, by extruding, a formable coating material to one or more surfaces of the profiled board, then roll the ornamental pattern into the material by contacting the coated board with a transversely-aligned impression roller, and permit the coating to harden. Whether using wood or coated composite moldings, the moldings are miter-cut to length and the miter-cut faces are slotted to receive a corner-joining clamp nail, and the corners are then fitted together and glued to form the completed frame. The frame may be painted, antiqued or otherwise surface-decorated prior to the cutting, slotting, and mitering operations.
The shaping of the wood to provide the desired profile and application of the ornamental surface represents a substantial portion of the cost of manufacturing frames. Various attempts have been made to reduce the manufacturing cost of frames. The principal one is the injection, transfer, compression molding or casting frames in their entirety in various plastic materials. In theory, the wood-shaping and application of the ornamental surface could be avoided by this technique and in the normal assembly of the frame pieces eliminated. However, because of the basic densities of plastics (60 to 80 lbs. per cubic foot) and/or the difficulty (in terms of molding time and, therefore, capital investment) in manufacturing thick sections, such frames have either been unacceptably heavy (except in very small sizes) or far more expensive than their wood counterparts. Furthermore, these have had a back hollowed out in such a fashion to give an unmistakable synthetic and "plastic" appearance and limberness, which has made them of considerably less value in the market place than an actual wood frame.
U.S. Pat. No. 3,268,636 discloses a lighter weight cellulated or foam structural plastic and a process for injection-molding the material therein to achieve molded bodies that are less dense (about 47 lbs. per cubic foot) than may be achieved with an injection-molded plastic and which also provides a relatively good surface finish. It was at first thought that the plastic process and machine disclosed in the said patent could achieve duplication of wood board as desired for use in production of frames. Early attempts to further reduce the density of the plastic to approximately that of wood (24-35 lbs. per cubic foot), while still retaining the other favorable properties of the process were unsuccessful. The lowest densities that could be achieved were about 40 lbs. per cubic foot, but then the inner cellular structure of the board-like product included very large internal voids which weakened the product and presented problems in fabricating the frame using standard frame manufacturing procedures and tooling. Furthermore, it was found that a good quality surface was difficult to achieve.
Relatively very low density replica wood beams can be manufactured in molds from urethane foam as disclosed in U.S. Pat. No. 3,561,181, wherein plastic of 2-6 lbs. per cubic foot is confined when molded to provide an average density of about 8 lbs. per cubic foot. However, such articles would not be suitable for the purposes of this application for use as moldings in that such construction would not provide the "heft" or weight which a frame purchaser expects, and such moldings are believed to be incapable of being integrated into existing frame production procedures without extensively changing the tooling and manufacturing procedures. While the same patent suggests foaming a more dense urethane plastic in open-sided molds to provide replica wood panels, it will be evident that the desirable features of a densified skin on all surfaces precisely replicating molding detail and surrounding a cellulated interior, as obtained by foaming under pressure with a totally confined mold, cannot be achieved.
Thus, an object of this invention is to provide a low-cost injection-molded cellulated board-like product which may be substituted for currently available wood or composite moldings in the manufacture of picture frames and the like, and which may be handled as a substitute wood product without changing other existing manufacturing procedures and equipment.
Another object of this invention is to provide a molding process whereby foamed thermoplastic board-like products can be produced which have a density similar to that of wood, of about 24.7 to 33.5 lbs. per cubic foot.
Yet another object is to provide molding equipment for use in manufacturing wood-like foamed thermoplastic boards.
Yet a further object is to provide a foamed thermoplastic board having an integral ornamental surface, which board is more economical to manufacture than previous constructions.
These and other objects of the invention will become apparent from the following description and appended claims.